1. Field of the Invention
The present invention relates to heat-curable coating compositions, also called powder coating materials, consisting of a carboxyl-functional polyester, and an epoxy resin and a special stabilizer.
2. Description of the Prior Art
Powder coating materials whose binders comprise carboxyl-functional polyesters and epoxy resins based on bisphenol A and epichlorohydrin, so-called hybrid powder coating materials, have already been known for a relatively long time; see e.g. T. A. Misev: Powder Coatings (Chemistry and Technology), 1991 and David A. Bates: The Sciences of Powder Coatings, 1990.
Powder coating materials based on carboxyl-functional polyesters and epoxy resins based on bisphenol A and epichlorohydrin possess outstanding mechanical properties but only little weathering stability and are therefore unsuitable for use in the exterior sector.
A disadvantage of powder coating materials whose binders comprise carboxyl-functional polyesters and epoxy resins based on bisphenol A and epichlorohydrin is, furthermore, the low yellowing resistance in directly gas- and/or oil-heated ovens in comparison to electrically heated ovens. In directly gas- and oil-heated ovens the coated parts are in direct contact with the combustion gases of the gas- and/or oil-heated burners during the curing operation. In powder coating materials whose binders comprise carboxyl-function polyesters and epoxy resins based on bisphenol A and epichlorohydrin this leads to relatively severe yellowing of the cured powder coating film in comparison to powder coating films cured in electrically heated ovens.